The invention relates to a method and mechanism for the thermal treatment of ore, and particularly for reducing and calcining lateritic nickel ore wherein the nickel ore is preheated by a stream of burning exhaust reduction gases to preheat the ore to reduction temperature.
Lateritic nickel ores occur as oxide ores, in which the nickel is dispersed through limonite, and silicate ores, in which the nickel occurs in a hydrated magnesium silicate.
It has been known in the reduction process for calcining lateritic nickel ore to use carbon monoxide and hydrogen at temperatures up to 800.degree. C. It has also heretofore been known to use kilns with overlying beds or multiple story furnaces with uncooled rabble arms. In this type of construction heretofore used, only small quantities of lateritic nickel ore may be given reduction treatment. The multiple story furnaces for larger charges have air-cooled rabble arms, and because of the possible uncontrolled air inlet, the danger of explosion exists and, therefore, this type of device is not suitable for the processing of nickel ore. Further, the processes do not make optimum use of the reduction gases. That is, when hydrogen is used, for example, as a reducing gas, the energy contained in the excess gas is disregarded.
A basic object of the invention is to improve upon methods and structures heretofore available while avoiding disadvantages such as those referred to above and providing a device and a method capable of a greater capacity and greater output.
In accordance with the principles of the present invention, the nickel ore is preheated to reduction temperature by an exhaust gas stream which is intermixed with air by swirling the stream through the supply of gas in a preheating zone. A strong reducing gas such as hydrogen is preferred, and after the hydrogen has been exposed to the preheated ore in a reduction zone, the still combustible hydrogen gas is burned obtaining additional heat from it, and this heat energy is used for preheating the nickel ore prior to its entering the reduction zone. The thermal treatment of the lateritic nickel ore takes place in a gastight system increasing its effectiveness and reducing the possibility of explosions.
An advantage obtained with the present invention is in obtaining an intensive reduction by the direct contact of the reducing gas with the nickel ore in the preheating stage so that the ore is uniformly and thoroughly heated to reduction temperature. A further advantage of the features of the present invention is that the energy contained in the reduction gas in excess of that utilized in the reduction process is utilized in combustion, and these combustible gases are used for preheating the nickel ore thereby obtaining a more efficient system saving in the cost of energy and increasing the capacity of an operative unit. As the reduction gases, such as hydrogen, are passed from the reduction zone, they are burned, and the heat or thermal energy being obtained from the reduction gases is mixed with the incoming ore in the closed system for a preheating operation. The reducing gases are heated in a burner chamber by being mixed with a tangentially swirling incoming burner which is also fed with the preheated ore so that the ore again contacts the reducing gases as they are being burned by the burner. The utilization of the heat contained in the reducing gases and by the burning gases of the burner is utilized by direct contact with the nickel ore through a series of preheating cyclones which are connected successively in series. This thermal treatment in the reduction zone, the burner zone, and the preheating zone is carried out in a gas-tight continuous system fed through a seal and discharged through a seal so that upon utilization of hydrogen in the reducing gas, no danger of explosion exists.
In accordance with a preferred embodiment, a reaction or reduction chamber into the which the reducing gas is fed discharges upwardly into a burner chamber, and in the burner chamber a burner discharges tangentially. The preheated ore is fed into the tangential burner so that a swirling intermixing occurs in the burner chamber. The reduction gas is initially fed into the reduction zone through a water cooled supporting grid or grate. A reduction or reaction chamber of this type for the treatment of finely grained material is disclosed in the German laid open specification No. 2,044,141. In the arrangement there disclosed, aluminum oxide is recrystallized and a reaction chamber is provided between a burner or a heated gas inlet with the surface of the whirling or turbulent layer following a wall which widens upwardly. The construction of the reaction chamber brings about a spatial separation of the cyclone turbulence produced by the heating gas and the turbulence layer coming up from the lower part of the reaction chamber. In that arrangement a mutual influencing between the cyclone turbulence and turbulence layer is prevented. It is made possible to arrange the reaction chamber so that optimum conditions are attained for the treatment of finely grained material in the turbulent cyclone and turbulent layer arrangement so that two reaction phases may be passed through by the material.
In the present arrangement an opening is formed through a constriction in the reaction chamber. The gases are introduced in the reaction chamber for the reduction process and then rise into the upper part of the reaction chamber and mix in the burning chamber with burning gases and combustion air. The reduction gases are then burned in the burner and continue to burn in an after burning chamber above the burner, and the thermal energy obtained from the reduction gases is then utilized in the preheating chamber.
In order to prevent the penetration of uncontrolled air and to prevent the danger of explosion, a gas sealed system is used, and a sealed material feed unit is utilized and a sealed material discharge unit is utilized. The process is thus basically a gas-tight arrangement so that no uncontrolled entry of air is possible.
The reducing gas, such as hydrogen, is fed in the reduction zone through a water cooled carrier grid or grate and whirls upwardly through the lateritic nickel ore lying above it and passing downwardly from the burner chamber. When the complete reduction has taken place, the finished material drops downwardly into a collection chamber below the grate, and is fed out through a gas-tight discharge mechanism such as a pressure feed worm conveyor. Pressure feed worm conveyors are used for the supply of ore and for the discharge of processed ore in that they provide a reliable operating seal, and by control of their speed of operation, a close control of the supply of material fed into the system can be attained.